kvm tools: Add 'kvm pause' command

This patch adds a 'kvm debug' command that's currently an alias for

  kill -USR2 `pidof kvm`

Which pauses a guest (freezes all VCPU threads) or resumes a paused
guest.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>